Frequency stabilization system



Feb. 20, 1951 s. l. RAMBO ET AL FREQUENCY STABILIZATION SYSTEM 2 Sheets-Sheet 1 Filed May 28, 1947 Feb. 20, 1951 s, 1, BO ETAL 2,542,837

FREQUENCY STABILIZATION SYSTEM Filed May 28, 1947 2 Sheets-Sheet 2 WITNESSES: Z9 INVENTORS I F" 5778/0 00 [Ea/0&0 000 EugpfLEurfip/jfi ATTOR NEY Patented Feb. 20, 1951 UNITED STATES I PATENT OFFICE asiass'z FREQUENCY STABILIZATION SYSTEM Sheldon I. Rambo, Baltimore, and Rupert L.

Rumpf, Jr., Catonsville, Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania.

Application 'May 28, 1947, Serial No. 750,970

13 Claims.

Our invention relates to electric discharge apis determined in part by the reactances associated with the oscillator, and in part by the reactance of the charge. In an electromagnetic heating system the charge reactance is inductive; in a dielectric heating system the charge reactance is capacitive. As the charge is heated and it temperature rises, its electrical properties change. The dielectric constant of a dielectric charge changes materially as its temperature increases. Similarly, the magnetic properties of a :ferrous charge changes over an extensive range as it passes through the Curie point.

The changes in the properties of the charge result in change in the reactances which determine the frequency of the oscillator and produce changes in the oscillator frequency. The oscillator frequency may also vary for other reasons such as wide fluctuations in ambient temperature or in the power supply.

other causes may result in violation of this condition. In certain situations, the electric discharge devices included in the oscillator are'designed to operate efficiently Within a limited frequency band. Frequency drift out of the band results in a material decrease in efficiency. The

peculiarities of the charge or the pecularitie's of the heating process also may require that the frequency of the oscillator be maintained within a frequency band. In such cases excessive drift of the frequency may not produce the result desired; The charge may be charred or insufliciently heated. ."rI-Iigh frequency heating equipment constructed in accordance with the teachings of the prior art, of which we are aware, includes a variable lumped reactance in the frequency determining network of the heating oscillator. The reactance iswariedautomatically .in responseto changes as to compensate for the variations.

in the frequency of the oscillatorin such a sense The power output of the heating oscillator is high and the potential differences impressed in the oscillator circuit are correspondingly high. Therefore, the dimensions of the variable reactance are large and its electrical inertia is high. As the react,- ance is varied in response to variations in the frequency of oscillation, it tends, by reason of its high electrical inertia, to overcompensa'te. A variation infrequency-in a sense oppositeto the original sense, which is nearly as large as the original variation; is produced. The reactance is then again varied and again overcompensates and the-above described compensating operation is repeated. This hunting process, once initiated by a variation infrequency. continues for an extended time interval and during the interval the operation of the oscillator is improper.

It is, accordingly, an object of our invention to provide a high frequency heating system, the frequency of whichshall be maintained precisely within a preset frequency band.

Another object of our invention i to provide a high power, high frequency oscillator for induction or dielectric heating purposes which shall include facilities for compensating for frequency variations and in the operation of which any tendency to hunt shall be suppressed.

A further object of our invent on is to provide a high power, high frequency oscillator including a frequency responsive tuning reactance of substantial electrical inertia, variation of which in response to changes in frequency shall not cause substantial overcompensation.

A general object of our invention is to provide a frequency compensated, stable source of high power, high frequency oscillations. I An ancillary object of'our invention is to provide a variable reactance of simple and inexpensive'structure for use in a high power, high frequency stabilized oscillator.

Another ancillary object of our invention i 'to provide a resonant network capable of operating in a high power, high frequency circuit and having high frequency stability.

In accordance with our invention, we provide a high frequency oscillator including in its frequency determining network a tuning reactance composed of a plurality of separately actuable components. Actuation of one of the components produces a relatively small change in the resonant frequency of the network. Actuation of several of the components produces a substan- 'tially largerchange in frequency. In operating may be efiected by movement oi cne oi theic'omponents of the reactance. As this one component has a relatively small electrical'iinertia, its actuation will not tend to cause overcompensa'tion. The several movable components of'the 'r'eactance have a substantial combined electrical inertia. If the frequency deviation is so iarge asflto re quire actuation of other movable components of. the reactance after the first has been actuated, overcompensation may occur. The motion of the *lar'g'erelectrical inertia 'is'started o'nly after-the smaller electrical inertia has to a large extent compensated for the original error. Therefore, the deviationresul'ting from the overcompensationj iss'iilcs'tantially smaller than the initial deviation. The overcompensation may then be "corrected by actuation in a-sense'opposite to that of the original actuation of the one component formers of the ordinary type. The primaries 25 and 26 are single loops which constitute the inductive reactanceof the resonant network 19. The secondary Si is a two-turn loop adjustably coupled to the primary 26, and the secondary 35 in a single-turn loop loosely coupled to the primary 25. The secondary 3|: may be, and frequently is, coupled to both loops'25 and 25.

The secondary 3| of the transformer 21 is connected to supply the load 33 or, in the case "oiwa highafrequency heating system, the charge is of the inductive type, the charge 33 is disposed ,so iaslvto :beprnagneticallycoupled to the secondwhich was actuated in response to the ;original "error. "Thenovel'ieatures that we consider characstic of our invention are set forthwith partion ion of the tuning react'anceused :inthe practice "6 our 'invention;

' "Figm l'is a view'in sectiontaken along'line IV--IV of Fig. 2";

' Ffi'g. "5is a view in elevation illustrating a inodifficat'i'on or our invention; and

' Figjo' i a view in 'secti'on ta kenalong the line vr vr of "Fig. 5. "Theanparatusshown'in'Fig. 1 comprises a high power, hi'gh'fiequehriy oscillation generator "'l including two electric discharge devices 9 "and I I, each having an anode i a cathode f5 and-a control electrode n. "Ordinarilyit is desirable "that-the frequency ofthe oscillator? be offthe 'cfder'of "10"or'='2'0"megacycl s. Under such circommittee the discharge "devices "Sand Fl fare of the high vacuum type, How'ever'in situations where the "frequency of "the oscillator is to be :substantian lower, electricf'discharg'e devices or -the gaseous type maybe-utilized.

Between the anodes of the dis'c'l'iarge devices,

lie-resonant network I9 is connected. *Thenetwork ='i1'1c'ludesthe two transformers 21 and" 2a, :the primaries 25 and :26 of which-:are connected series, a :fixed capacitor 2 I connected between the router terminals or the primaries 25,i:and a pair of ."sim'ultaneously variable serially connected capacitors 29 and 40 whichiare :oonn'ected between theasame terminals.

cu rity in the appended claims; "The 'inven-i v self; however, both "as'to its organization a .{and 'its method *of operation'together with addij- 1g. '3'is a view in end elevation showin g'a porary. The secondary of the othertransformer 23"is'coupled to a frequency compensation systern. The load transformer 2! is designed to carry substantial power; the compensation transformer 23 is of the low a power type.

The structure of the capacitors :21! and '29za'nd "their connection to the 'dischargeidevices 9 and l l are shown in Figs. 2 to 4. "Each discharge clev'ice is 'of the type having a hollow cylindrical un'e'tall'ic jacket 31 through which acoolingi'fluid such as water is conducted. The inner wall of the "jacket 31 constitutes the anode 3 of each device. The fixed capacitor 2'! has fi xed hori- "zontal' plates 'ss which are conductively-secured to the outer cylindrical walls :of the fjacketsSl; the f-151fit'88- secured to one of the jackets overlapping the other plates. The variablecapaeitors Ziland H] have sets of spaced horizontal-fixed plates 4'! which are :of rectangular shape and are 'asse'mbled on two vertical rectangular plates The two vertical rectangular plates 43 :are suspended from theibasesxofthe jackets 3'i,:and each vertical plate lconductive'ly carries a set if spacedihorizontal fixed plates-" l Alternatively,- th'e'fixed plate assembly could have been constructed integral with the fixed capacitor; .in which case, the plates 39 and '4! *wouldhaveibeen "mountedona bracket which is :securedto the jackets 13?. In the assembly shown, the vertical supporting-plates 43 are secured 'bywelding :or ibrazing tothe-jackets 31, orstheypmay-be bolted tozthe jackets.

The two variable capacitors "29 and 43 have a common-set .of movableplates'flfi of butterfly shape. The movable plates liSaretsecriredzt-o a "plurality of collars 51,49 and 5!. The collarand plate assemblies are stacked on avertical shaft 53 which is rotatable from a? motor '55. The 'top (collard? has a short transverse. slot 51,1the next the shaft. The pinsare then insertedtthrough the slots-in holes 'provided'in the shaft.

' f The'slot 51 in. theiupper'collarf l'i has a length :only slightly greater than the diameter :of its 'ipin :63. accordingly; as; the {shaft 53 is rotated theupper movable condenser plate Q43 rotates V anditheloWer plate. rotates withit afterithas Lirotated through ,axsubstantially :g'reater 1 angle.

Since the movable plates 43 of the capacitor 29 each have a butterfly shape, the variable capacitor varies in a sense dependent on the direction of rotation as the shaft 53 rotatesl Initially, the variation is at predetermined rate as the upper plate alone is rotated; the rate of variation is increased substantially when both the center plate and upper plate rotate; and a further substantial increase in the rate of variation is introduced when the lower plate isrotated with the others.

The compensating and load transformers 2I and 23, the load 33, the fixed capacitor and the variable capacitors 29 and 40 constitute a resonant network which determines the frequency of oscillation of the oscillator I. The control electrode I! of one of the discharge devices 9 is connected to its cathode I5 through a choke 65, a variable resistor 61 and the exciting coil 69 of an overload relay 'II The cathode I 5 of this discharge device 9 is returned to ground through the exciting coil I3 of a second overload relay I5. The control electrode of the other discharge device II is connected to its cathode through another choke II, a fixed resistor I9, the exciting coils BI and 83 of two relays 85 and 81, respectively, and the exciting coil 89 of an overload relay 9|. The latter cathode I5 is also returned to ground through the exciting coil 93 of a fourth overload relay 95. .The junction between the choke 55 and the variable resistor 61 is directly connected to the junction between the choke I1 and the fixed resistor I9. Between each of the grids I! and its associated cathode I5, the network consisting of the variable resistor 61, the exciting coils 69, 13,93 and 89 of the overload relays II, I5. 95 and 9|, respectively, (the exciting coils 83 and BI of the relays 81 and 85, respectively, and the fixed resistor I9 is connected.

The oscillations in the generator 'I are built up by feedback through the interelectrode capacities between the anodes I3 and the grids I5 of the discharge devices 9 and II. The oscillatory potential across the resonant network I at any instant is increa ing at one terminal (the anode of one tube) and decreasing at the other. At this instant, the potential of the anode I3 at the former terminal is becoming more positive while the anode I9 at the other terminal is becoming more negative. The grids H of the corresponding electric discharge devices 9 or II become respectively less positive and more positive as the anodes become more positive and le s positive thus further increasing and decreasing the respective positive potentials of the correspond ng anodes. The oscillation is built up until the build-up action is reversed by the restoring'action of the resonant network I. When the reversal occurs, the function of the anodes and grids of the tubes 9 and I I is interchanged. Since the variation of the anode potential is always of opposite polarity to the variation of the grid potential of the same device 9 or H, the oscillations in the circuit are built up and sustained.

As the circuit "I oscillates, one or the other of the control electrodes I1 is at a positive potential relative to its corresponding cathode I5. Grid current therefore flows through the corresponding discharge device. The grid current of the discharge device 9 flows through the parallel branches: one made up of the variable resistor 61 and the exciting coil 69 of the overload relay 'II; the other of the fixed resistor I9, the exciting coils BI and 83 of the relays 85 and 81 and the exciting coils 89, 93 and 13 of the other overload relays 9|, 95 and I5. The grid current of the other discharge device II flows through corresponding branches of the same network. If the grid current in either device 9 or II exceeds a predetermined magnitude, one of the relays, 85, is energized, opening its normally closed contacts 91. Variable resistor 61 is then increased and it tends to suppress the excess grid current. If the grid current is still greater than the predetermined magnitude, the other relay 81 i also energized and the resistor 61 is further increased. The increase in the variable resistor 61 of the network results in a decrease in grid current. However, the grid current through the branch of the network in which the exciting coils BI and 83 of the relays 85 and 81 are connected is not changed materially, and once either or both of the relays is actuated they remain actuated.

For the purpose of stabilization, the output of the oscillator I is compared with oscillations of a standard frequency derived from a crystal controlled oscillator 99. The latter is provided with a pair of resonant output circuits IM and I03; one (IOI) tuned to its fundamental frequency and the other (I03) tuned to a harmonic frequency. Depending on the frequency at which the power supply oscillator I is to operate, one or the other of the resonant networks is connected to the compensating circuit through a selector switch. I05.

The selected output of the crystal oscillator 99 is connected through a capacitor I01 to one of the grids I09 of a mixer tube III and through a resistor II3 to the corresponding grid II5 of a second mixer tube Ill. The capacitor I01 and the resistor I I3 are so related that there is a phase displacement of between the control potentials impressed on the grids I09 and H5, respec tively.

A sample of the output of the supply oscillator 1 derived from the compensating transformer 23 is impressed on other corresponding grids H9 and IZI of the mixer tubes III and Ill, respectively, through a coupling condenser I23. In one of the mixer tubes III, the sample output of the supply oscillator I is combined with the output of the standard oscillator 99; in the other mixer tube, the sample output is combined with the output of the standard oscillator displaced in phase by 90 with reference to the standard output which was put into the first mixer tube.

From the output of the mixer tubes, oscillations having a frequency equal to the difference between the combined frequencies are derived. Between the oscillations derived from one mixer tube and those derived from the other, there exists a phase displacement of 90 of the standard frequency, which would be a very small phase d splacement at the difference-frequency. The output of each mixer. tube I09 and II! is impressed in the control circuit of a separate amplifier tube I25 and I2'I, respectively, through a coupling condenser I29 and I3l, respectively.

The output of one of these amplifiers I25 is impressed in the control circuit of a tube I33 connected in a phase splitter circuit through a coupling capacitor I35. Between the cathode I3'I of the phase splitter tube I33 and ground, a pair of resistors I39 and I 4| are connected in series. The anode I43 of the phase splitter tube I33 is connected to one of the anodes I45 of a double diode I4! through a capacitor I 59. The junction between the seriesresistors I39 and MI is connected to the other anode I 5| of the diode I4] through:anotherzcapacitor 1]"531Dfi1bh6 asamet'mag mtude asithe capacitorfzlltQ. Between the anodes Mtrand 151 ofiithe diode, alpair of resistors 1 .515 and [55? of equal magnitude are connected. ;Be-. tween the cathodes l'59and H31 of thGLdiOdBi'Al', another pair of resistors 163 and 1.65 of :equal magnitude are-connected. The junction between the resistors i155 and 1 .51., and the junction between the resistors 163' and 165, are connected, as shown at .126 i.

r The potentials produced :at the anode 143 of the phase splitter tube 1'! 33,:as the "oscillatory output of the amplifier 125*15 impressed in its :controkcircuit,.isrinnpposite phasexto the correspondingpotentials impressed at the junction between the resistors i139 :and 'l' lfl in its cathode circuit. Thepotentials impressed between the anodes I45 and 1.5:! of the diode 4'49 arethusin opposite phase.

"The Glltplit of the second amplifier- 1 21 is coupled to conductor 1 6'! interconnecting the junctions between the resistors 15%; and 1-51, and :i'83tand' H65, respectively, between the anodes 145 and i251 and the cathodes 159 and 161 of the diode :I -ii. This output is in quadraturewith the opposite phase potentials derived from the phase splitter.

Through the phase splitter "l 33- and the second amplifier i527 the .resultant' of three vector potentials is-impressed between the anodes and the cathodes of the diode; that is, across the resistors W3 and-165' interconnecting the cathodes of the diode; When the difierence between the stand-- .ard frequency andthesample irequency'is within prescribed limits, the opposite phase potentials andthe potential derived from the :second amp ifier 12-5 are in precise phase quadrature, and the potential'drop across the resistors-connected between the cathodes of the diode is substantially zero. The capacitors I49 and I53, .the resistors I55 and 151, and the impedances of thediode are so related that when the frequency difference between the sample'oscillations and the standard oscillations varies from the prescribedmagnitude. the opposite-phase potentials shift in phase with reference to the potential outputof the second amplifier I25. The output of one or the other of the diode .halves then predominates and a potential .of one polarity or the other is impressed between the-resistors I63 and [6.5 interconnecting the cathodes 159 and it oi'the diode.

We have found that for supply oscillator frequencies of the order of or megacycles, the diode l4! may be a 6H6 tube the capacitors interposed between the phase splitter and the anodes The cathodes its and as: of the diode are 'eachconnected directly 'to the control electrode l't and ill, respectively, of a separate amplifier H3 and IE5, respectively. The anodes H1 and I19 of these amplifiers are connected to a source of potential lill (shown symbolically as a'battery) through one of the two exciting coils I83 and 185, respectively, of a polarized *reay 181. When the potential between the resistors m3 and I55. interconnecting the cathodes I59 and I6! of the diode It! has one polarity or the other, one orthe other of the amplifiers H3 or 115 becomes conductive and the corresponding coil 183 or 1'85 of the polarized relay I8? 'is energized. This relay actuated to close oneorthe other of its-circuits toenerg izethe motor -55 to rotaterthe' movable plates :41 of the variable condenser 2.9, in .one

" direction or the other.

To initiate operation of the power oscillator. arstartingswitch 189 is closed. The switch closes a circuit extending from a power source-I311. (shown symboically as a battery-)' through the.

exciting coil 193 of a starting relay I95, a con-.-

ductor I91, the normally closedcontacts l99,.2l1l., 20.3 :and 205 of the overload relays M, 95, 15 and H, respectively, in the control and cathode cir-v cuits of the oscillator discharge devices Band H, aconductor. 2&1 {13011116 switch.

The starting relay- ];95 is actuated opening its lower contacts 269 .and closing its upper contacts 211-.- Eower .issupplied through the latter. .con-

.tacts..2.l1l froma source .213 (shown symbolica'ly the cathode .2l9 of which a resonant network 22! .is connected through a negative bias .223. The. networkc22l is tuned to. the center frequency of the band within which it is desired.that the oscillator "i. operate. The output of the compensating transformer 231s coupled to thene'twork 22!. In theanode circuit of the discharge device 215 the exciting coil..225 of a relay '22! is connected. v

If the frequency of the oscillator I .lies outside of. the compensating range, the potential im-. pressed in the resonant network 22! is relatively small, the discharge device is non-conductive and the re1ay"22l is deenergized. The mixers HI and .I H are aso unaffectedby the sample output from. the supply oscillator l and the polarized relay [8! is deenergized. The movable contactor 229 of the latter relay is disengaged from its fixed contacts. V

The motor '55 is provided with a limit switch 23! having a movable contact 233 and fixed contacts23'5 and 231. The switch remains closed in one or the other of its positions when the mov able plates of the variable capacitor 29 are in an intermediate angular position. When the plates 45 reach a limiting position, the movable contact 233 is actuatcdby a earn 238 to dis engage one'fixed contact 235 (say) and to en gage the other 237. In the drawings, the limit switch 231 illustrated as of the cam typef 'However in 'lieu of-a cam switch, a toggle switch may be provided.

With the limit switch 23! in the'position shown in'the drawings, the starting relay I is actuated and the frequency of the oscillator beyond the operable limits of the compensating circuit "(the relay'Z-Z'J in thecorrecting circuit in the position shown), current flows from one terminal of the power source 13! through a resistor 239, a con ductor'2 ii, the fixed and movable contacts 235 and 2:33 of 'the'limit switch, a conductor 243, the normally closed contacts 245 of the-relay 221; intermediate contacts 2 il' or the starting relay closed "by actuation of the star-tingswitch, the exciting coil 259 of one of a pair of relays 25B sunset and 253, controlling the direction of rotation of the motor 55, to ground. The relay 25! is energized opening its lower closed contacts 255 and closing its upper contacts 251. At the same time the exciting coil 259 of the other relay 253 controlling the direction of rotation of the motor 55 is short circuited by the limit switch 23! and remains deenergized.

Current nOW flows from one terminal of a power source 25! through the normally closed contacts 253 of the deenergized controlling relay 253, a conductor 265, the rotor of the motor 55, a resistor 261, the closed contacts 251 of the energized controlling relay 25!, t the otherterminal of the source The motor 55 rotates in one direction varying the frequency of the oscillator 1 in one sense and, if the variationis in the proper sense, the oscillator 1 is locked in with the compensating system.

The variation of the oscillator frequency, however, may not be in the proper sense to bring the frequency of the power oscillator 1 within the range of compensation. Under such circumstances the correcting relay 221 remains deenergized and the motor 55 continues to rotate until the limit switch 23! is actuated to its opposite position. The exciting coil 249 of the direction controlling relay 25! which is at this time energized now becomes deenergized, and the exciting coil 259 of the other controlling relay is energized in a circuit extending from the power source !8! through the resistor 239, a conductor 259, the exciting coil 259 of the controlling relay 253, the intermediate closed contacts 241 of the starting relay I95, a conductor 21!, the normally closed contacts 245 of the correcting relay 221, the conductor 243, the contacts 233 and 235 of the limit switch to ground. The motor 55 is now rotated in the opposite direction. The capacity of the tuning condenser 29 is Varied over its entire range and during a portion of the rotation of the motor the supply oscillator 1 oscillates within the range of compensation. During this portion of the rotation, a potential of substantial magnitude is impressed across the resonant network 22! in the control circuit and of the correcting discharge device 2!5. The correcting relay 221 is now actuated, its lower closed contacts 245 open and its upper contacts 213 close.

The exciting coils 249 and 259 of the controlling relays 25! and 253 are now disconnected from the limit switch 23! and connected to the movable contact 229 of the polarized relay I81. .As the polarized relay contacts close on one side or the other, the motor 55 rotates in one direction or the other. For example, if the contacts on the left are closed, the exciting coil 259 of one of the control relays 253 is short circuited while current is supplied to the exciting coil 49 of the other. The current flows in a circuit extending from the source !8! through resistor 239, a conductor 215, the closed contacts 229 and 211 of a polarized relay !81,'a conductor 219, the upper closed contacts 213 of the correcting relay'221, the conductor 21!, the intermediate closed contacts 241 of the starting relay !95, the exciting coil 249 of the controlling relay 25! to ground. The controlling relay 25! is now actuated, while the relay 253 with its coil 259 short circuited remains deenergized, and the motor rotates in such a direction as to correct any variation in frequency of the power oscillator 1.

The system shown in Figs. 1 to 4 is provided with a circuit, which to a certain extent prevents itial compensation is in one sense. This circuit comprises an electric discharge device 28!, preferably of the gaseous type. The anode 283 of the discharge device 28! is connected to one terminal of the rotor of the motor through analternating current supply source 285 and a rheostat 231. The cathode 289 is connected to the opposite terminal of the rotor. The control potential of the discharge device is dependent on the position of one of the direction controlling relays 25!. With the controlling relay 25! de--v energized, a negative control potential is impressed on the discharge device 28 and it is nonconductive.

When during the course of operation the con: trolling relay 25! is energized, its lower auxiliary contacts 29! open and its upper auxiliary contacts 293 close. A charging circuit for a con-' denser 295 in the control circuit of the discharge device is closed through the latter contacts 293;

' However, the closing of the condenser charging circuit does not materially aiiect the control pee tential of the discharge device 28! and the latter remains cleenergized. The motor 55 rotates in a sense determined by the relay 25! and the IeSQj; nant frequency of the supply oscillator is varied; When the tuning capacitor 29 becomes set so that the frequency of the oscillator 1 is within, the required band, the contacts 229 and 211 of the polarized relay !81 through which the controlling ity of the current flow through theresistor 291 is such as'to render the discharge device conduc-. tive.

overcompensates.

= We have found that the anti-hunting circuit. including discharge device 285 does not, ifincluded in prior art systems, entirelysuppressovercompensation. The difiiculty arises from the fact that the inertia of the tuning capacitor in, such systems-is so large as to maintain the rota-. tion of the motor. The overcompensation is,

however, effectively suppressed in a system in ac cordance withour invention.

the'motor frompvercompensating when the ini 'For an understanding of the operation of our system, we may assume that the movable plate assembly of the tuning capacitor 29 is in a posiv tion such that the pins 63 on the shaft 53 engage, the sides ofthe three slots 51, 59 and 5!., We may also assume that when the polarized relay,

I9! is closed the direction of rotation of the shaft 53 is such that all three plates 45 rotate simul-f taneouslyr The plates 45 will continue to rotate until the frequency of the supply oscillator 1 is within the required band. By reason of theirin-- ertia, they will then continue to rotate so that the frequency of. the oscillator 1 eventually falls outside of the'band in a'sense opposite the orig--' inal sense. The other contacts of the polarized-- relay !81 will now close and the rotation of the movable plates 45 of the capacitor 29 will be reversed. However, only theupper plate 45; of, the; capacitor willnowrotates The inertia of this plate being substantially smaller than the in ertia of the three plates 45,- the tendency to overcompensate will be materially reduced and the power oscillator will be maintained Within the desired frequency band. The foregoing descrip relay 25! is energized open and the controlling;

The condenser 295 is now discharged:

The latter substantially short-circuits the; rotor and tends to stop the motor 55 before it.

Iii tion of the compensating-effect covers i only one mode of operation .of the system. Other modes of operation within the scope of our invention will be apparent to one skilledin the. art.

In the customary practice of our invention, a series of charges are repeatedly heated by the poweroscillator. 'For example, a series of dielectricdiscs may besuccessivelypassed through heating condenser plates during the course of amanufacturing process. Alternatively a series of pin's orgears may be passed througha heating coil successively for heat treatment.

compartment, the starting switch l89-isturned onand off. During the time interval during which'the charge vis subjected to the electromagnetic field within the-heating compartment, itsproperties vary materially. The reactanceof thec'harge and'the'frequency'of the power oscillator-therefore vary materially. The frequency correction is affected by varying the tuning capacitor 2'9. Since the reactanceof the "load- 33 'is difierent at the end of the heating operation than-at the beginning-of a heating operat-ion,

themagnitud'e of the-capacity 29'isdifferent at the endof 'a'heating operation than'at-the beginning of anoperation. 'It' is desirable that "for each-=new heating operation, the -oscillator- -be so set that its frequency'is immediately substantially within the requiredrange at the beginning of the operation.

"To'accomplish this objective, the tuning-capacitor is reset following the end of each heat ingoperation by --operation-'of"a Wheats-tone bridge 298. The bridge comprises-fixed resistors 2-H9 and *301, a rheostat 303' and a "resistor "305 which varies with the rotation of the motor-"55 connected in the usual series-network. A power source 301- is connected across two'ofthe termi nals of the network and "a polarized relay 309 acres the other terminals.

Initially several trial operations'are'conducted and the'rheostat'30-3 issetso' that the relay 3'09 is-balanced atthe desired--value zit'the b'eginning of each operation the starting relay I95 is energized'and, because its lower contacts 209' are opened; the polarized relay in -'the'balancednetwork is "disconnected from the direction-com trolling *relays 25'l and-253. "As -the movable direction or the other'untilt'hebridge298 is" V balanced. At the same'time, the tuning-capacitor-29*is returned to'i-ts initial magnitude; For example, if the contacts 3| I "and-3H onthe' left or the polarized relay 309 are closed, current is supplied from the'power'source, l-8=l "through the closed'contacts '3 l I i and 3 l 3, a conductor3t5, the now reclosed lower contacts-2 ll9 'Of thQ-staIting 'relay '|9|,the exciting coil '24'9-of"the'c'ontrollingrela-y 251 to ground. 'The other controlling re1ay'253 isactuated in a-corresponding manner;

As the successive items move in and out of the heating 'IInfFigs. .5. and 6, a modified tuning-capacitor in accordance withour invention is shown. latter comprises a 'pair of: fixed plates MT- and 319 in the form of arcs of a 'cylinderwithiwhi'ch a pair of movable plates 32| and s23 of similar form cooperate to produce avariable capacity; Each fixed plate '31- and'3l9 may be welded or brazed to the outer wall of a jacket of a-discharge device (not shown) in-an oscillatorcircuit. One of the movable plates323ismounted to rotate with ashaft'325' which may be rotated from the motor (not shown) through a system of gears. The other movable pla'te is mounted on another shaft 321. This shaft 32'l is inserted in a-oollar 329 secured to the'first mentioned shaft 325 so that it (the collar) rotatesswith the latter. "The collar'is provided with a slot 33l of substantial width' which is engage'd' 'bya pin 333 mounted on the shaft 32! inserted in'its end. When'the motor is energized, it initially-rotates the mo'vable plate 323 to vary the capacitor at'a predetermined rate. After the shaft 3-21 has rotated through a, predetermined angle, the pin 333 engaged'by the wallsofthe slot=33l in the rotating collar 3'29 and the other movable plate 32 i is rotated'so that the capacitor is varied at a substantially greater rate.

'Although we have shown and described certain specific embodiments of our invention, we are fully aware that many modifications thereof are possible. Our invention, therefore, is 'not'to be;

restricted exceptinsofar'a's 'is'necessitated by the prior art and by the spirit of the appended claims. I

'We'cl'aimas'our'invention;

1. An'oscillator comprising'a'valve and'a'resonant network for determining the frequency of oscillation of said oscillator, said "resonant network including a reactanceand a moving mechanism responsiveto'the frequency of said'oscil lator for varying said reactancethe movement of said mechanism in one sense varying said reactance in one-senseat a rate which is a'func tion of the extent of movement untilsaid move,- ment reachesa predetermined extent'and movement of said. mechanism. beyond said. extent varying said reactances at a substantially higher rate as a function of vmovement.

2. In combination, .a resonant "oscillatory net- T workincluding areactance, anda movingmechanism responsive Itolthe frequency of said network .for varying said, reactance to varythe resonant frequency of said network, saidreac tance being composed of .a plurality-of sections relatively movable by said mechanism so7that the movement of said mechanism in one sensevaries saidreactance at apredeterminedrate' as a"func-' tion of theextent ofmovementuntil said movement reaches aipre'determine'd extent 'and'movement of said mechanism beyond said Iext'ent varying Isaid reactance "at a substantially'higher rate as aifunotion of movement of said mechanism. V

.3. In combinatioma resonant oscillatory'net work including a condenser and a moving mechanism responsive to theirequency of said network for varying. said .condenser to varythe resonant frequency of said 'network said condenser having fixed plates. .andyplates; movable relativeto said. fixed platesb said. mechanism, certain of said movable plates being the .only "plates movable by said mechanism until the movement ofsaid mechanism reaches a'predetermined extent, and as the movement of said 'mechanism l3 extends beyond said extent certain other of said plate being movable by said mechanism. 4. An oscillator, including a valve and a resonant network for determining the frequency of oscillation of said oscillator, said network including a reactance, said reactance being composed of a plurality of relatively movable sections, and a moving mechanism responsive to thefrequency of said oscillator for moving certain of said section relative to the others so that the movement of said mechanism in one sense varies said reactance at a predetermined rate until said movement reaches a predetermined extent and movement of said mechanism beyond said extent varyin said reactance at a substantially higher rate as a function of movement of said mechanism.

5. In combination, a resonant oscillatory network including a condenser and a moving mechanism responsive to the frequency of said network for varying said condenser to vary the resonant frequency of said network, said condenser having fixed plates and plates movable relative to said fixed plates by aid mechanism, certain of said movable plates being the only plates movable by said mechanism until the movement of said mechanism reaches a predetermined extent, and as the movement of said mechanism extends beyond said extent certain others of said plates being movable by said mechanism, together with said first-named movable plates.

6. An oscillator comprising a valve and a resonant network for determining the frequency of oscillation of said oscillator, said network including a condenser having fixed plates and plates movable relative to said fixed plates, certain of said movable plates being movable relative to others of said movable plates, and a mechanism responsive to the frequency of said oscillator for moving said movable plates, coupling means connecting said mechanism to said movable plates to move certain of said movable plates only in one sense until the movement in said sense reaches a predetermined extent and to move certain others of said movable plates in said one sense beyond said predetermined extent.

7. An oscillator comprising a valve and a resonant network for determining the frequency of oscillation of said oscillator, said network including a condenser having fixed. plates and plates movable relative to said fixed plates, certain of said movable plates being movable relative to others of said movable plates, and a mechanism responsive to the frequency of said oscillator for moving said movable plates, coupling means connecting said mechanism to said movable plates to move certain of said movable plates only in one sense until the movement in said sense reaches a predetermined extent and to move certain others of said movable plates together with said first-named movable plates in said one sense beyond said predetermined extent.

8. A frequency stabilizing system comprising a resonant network, a variable reactance for varying the resonant frequency of said network, said reactance being variable in sections, variation of certain of said sections producin variations in the frequency of said net-work to a greater extent than variations of other of said sections, a moving mechanism for varying said reactance, movement of said mechanism in one sense varying said first-named sections to vary said frequency at a predetermined. rate as a function of extent of movement until said movement reaches a predetermined extent and movement of said mechanism beyond said. extent varying said other sections to vary said frequency at a substantially greater rate as a function of extent of movement and a controller for said mechanism, responsive to the resonant frequency of said network when it varies from a predetermined frequency range, to cause said mechanism to move'to correct for the variation.

9. A frequency stabilizer comprising a frequency determining network and a controller responsive to the resonant frequency of said network when it varies from a predetermined range of frequencies to return said network to a resonant frequency within said range, said controller operating to vary the frequency of said network from its initial value in a predetermined sense by predetermined increments until the varia-- tion from the initial value reaches a predetermined magnitude and to vary said frequency in said sense by substantially greater increments when the variation from said initial value exceeds said magnitude.

10. A frequency stabilizer comprising a frequency determining network and a controller responsive to the resonant frequency of said network when it varies from a predetermined range of frequencies to return said network to a resonant frequency within said range, said controller operating to vary the frequency of said network from its initial value in a predetermined sense by predetermined increments until the variation from the initial value reaches a predetermined magnitude exceeding the magnitude required to return said network to said resonant frequency and to vary said frequency in the opposite sense by increments substantially smaller than said predetermined increments to return the frequency of said network to said predetermined range. a

11. A frequency stabilizing system comprising a resonant network including a reactance having fixed plates and movable plates, certain of said movable plates being movable relative to certain others of said movable plates, a controller, responsive to the frequency of said network when said frequency is at a magnitude outside of a predetermined range of frequencies, operating to move only said first-named movable plates in one sense such as to return the frequency to said range until the frequency of said network varies from said magnitude by a predetermined quantity and to move said other plates in addition to said first-named movable plates in said one sense when said quantity is exceeded.

12. A frequency stabilizing system comprising a resonant network including a reactance having fixed plates and movable plates, certain of said movable plates being movable relative to certain others of said movable plates, a controller, responsive to the frequency of said network when said frequency is of a magnitude outside of a predetermined range of frequencies, operating to move said first-named and said other movable plates in one sense such as to return the frequency to said range until the frequency of said network varies from said magnitude by a predetermined quantity exceeding the quantity which would have returned said frequency to said range and to move only said first-named movable plates in the opposite sense to return the frequency of said network to said range.

13. An oscillator comprising a pair of tubes l -5 each having-,amanedemnd as-reseriant :netwbrk'for determining the irequency of .oscillation inf :said oscillator, said resonant network including "a reactance, ,saidreactance comprising fixewplates physically attached to the arieties ofsaid tubes, movableplatesi intermeshed' with said-fixed plates, said movab1e;.p1ates being 'ca rriedzon a shaft'in such manner that as saidzs'haft is rotated-said movable-plates are,progressively :moved :a;predetermin'ed extent,:,a moving mechanism-responsive REFERENCES CITED The-"following references are of-recortl-i'n the file of this I patent:

Number- 1,550,016

Number rUNIFIED-QSTATES 7 "Name Date- Dodge Aug. 18, 1 925 Clinker Sept. "22', 1925 Hayden Mar. 22, 1'92"; Marvel "July' 1,1930 Washington May' 30,1'933 "Hurt Nov. 28,1933 Appel, 'Jr. Jan. 9,1934 Reinken May 21, 1935 Usse'lman June 16, 1936 Carpenter Apr. 26; 1938 Farrington June'18, 1940 .FOREIGN PATENTS Country Date Great Britain July 20,1926 

